The present invention relates a process for producing polyester fibers of high tenacity and high modulus.
At present, polyesters, representative of which is polyethylene terephthalate, are widely used for fibers, films, bottles, plastics, etc. The improvement of mechanical properties and performance of these shaped articles is strongly desired in their respective fields. It has been known for a long time, as stated by H. F. Mark, "Man-made Fibers" (Science and Technology), Vol. 3, page 12 (1968), Interscience Publishers, that the mechanical properties of a polymer are closely related to the polymer's own molecular weight. The present invention relates to a process for giving polyesters an ultrahigh molecular weight by a new means called polymerization in a heating medium, and for obtaining fibers of high tenacity and high modulus.
In general, polyester fibers are produced by first obtaining glycol ester and/or its low molecular weight polymer by direct esterification of an aromatic dicarboxylic acid and glycol or by ester interchange of an aromatic dicarboxylic acid alkyl ester and glycol, and then obtaining a polycondensation product by heating them with stirring under a high vacuum, followed by melt-spinning of the polycondensation product and drawing the resulting filaments.
On the other hand, in Sen-i-Gakkaishi, Vol. 35, No. 8, T-328 (1979), an attempt is disclosesd, wherein drawn polyethylene terephthalate filaments are heat-treated in filament form to give them a high molecular weight by solid state polymerization. However, both tenacity and modulus are lowered with the heat treatment, and therefore such an attempt is not related to the attempt of obtaining fibers of high tenacity and high modulus aimed at in the present invention.
The method of polycondensation by heating with stirring under a high vacuum is widely adopted industrially at present. But this method requires a vacuum apparatus for maintaining a high vacuum, and a strong power for stirring a highly viscous substance, and consequently the apparatus has to be complicated and a high cost is necessary. In the use for industrial materials in particular, polymers of a higher degree of polymerization are required, and in general, after the melt-polymerization as mentioned above, the resulting polymer is made into chips, and then the chips are subjected to solid state polymerization by heating for a long time in an inert gas or under vacuum, thereby to obtain a polyester of a high degree of polymerization. However, by this method, there is a limitation in the degree of polymerization of the resulting polyester. We therefore searched for a polyester of a higher degree of polymerization. As a result, we have succeeded in obtaining a polyester of an ultrahigh molecular weight which has not been obtained by the conventional methods, by a new method called polymerization in a heating medium. As used herein the term "polymerization in a heating medium" means polycondensation of oligoester or polyester in a heating medium by heating with stirring while blowing an inert gas into the medium.
Although it is possible by this polymerization method in a heating medium to obtain an ultrahigh molecular weight polyester that has never been obtained, the viscosity of the resulting polymer is too high and accordingly it was extremely difficult to give fibers high tenacity and high modulus by adopting the conventional melt-spinning method.